How to switch all lights on/off at once in Cycles?
You could just use instances. (Copying with Alt + D)
Or, after the fact, assign the same Data Block to every light here:



Alternative with Drivers:

Add a driver like this to the Strength of every lamp. That way you can maintain more control over individual lamps.



This uses the zLocation of an Empty and multiplies it with the Strength. You would simply use the same Empty for all Lamps. That way you can control them all at once.

Update for Color

Here's an example to control the color in a similar way.

This is the node setup for the Lamp.



Then add the following drivers accordingly.





This is relatively intuitive to use.